It is a well-known physical phenomenon that electricity generation in systems using a galvanic operating principle is made possible by the free-ion conductivity of salts dissolved in water. Research targeted at harnessing—as an electrolyte—the inexhaustible water supply of seas and oceans for the generation of electricity in an efficient and economical manner has been going on for a long time.
Compared to the acids and alkalis (bases) applied in galvanic systems (batteries), seawater is a lot more dilute and is a ‘lower-level’ electrolyte, which has a great influence on the level of power that can be generated.
In energy cells using the galvanic operating principle, the application of an anode, a cathode, and an electrolyte that match one another is required for economic operation. In the case of the application of seawater as electrolyte it is not possible to modify the composition of the electrolyte, and furthermore, it is strictly prohibited to pollute seawater in any way, i.e. such systems must not have any harmful effect on the environment. Accordingly, research has been focusing on improving the anode and the cathode.
One of the known approaches is the MetalCell battery that can be recharged with saltwater. This development was originally started with military application in mind. It is a small-size emergency battery that is capable of powering a laptop for a few hours. When recharged with saltwater, the battery can be used again but it has a restricted service life, since the magnesium—applied as the anode of the battery—degrades over time. In a dry state the battery can be stored for indefinite time. It needs to be charged only immediately before it is put to use. This development was not directed towards supplying energy on a larger scale; the MetalCell battery has limited dimensions.
A magnesium-air fuel cell is also known. Essentially, this technology employs a vessel made of a gas-diffusion material (cathode) which is filled with seawater, with a magnesium rod (anode) being placed into it. Under the influence of oxygen flowing at the cathode's surface, an oxidation process is started on the anode, which results in an electric current being induced between the anode and the cathode. This method has the drawback that the cells have to be physically separated, and that it can only be applied on free air, in a dry environment.
A seawater battery for powering electric torpedoes has also been developed. The anode material is predominantly magnesium, while the cathode is silver chloride applied to a silver film. The system is capable of producing very high power levels, reaching even 500 mA/cm2, but discharge time is at most 10-15 minutes. It cannot be recharged after use. Because of the silver it has very high production costs.
In the so-called Dunk battery the electrolyte is also seawater and the anode is substantially of magnesium, but the cathode is copper chloride, a material that is harmful to sea environment. Dunk-type batteries are manufactured in a conventional configuration, with physically separated cells. The voltage level of a single cell is 1.5 V. Discharge time is 0.5-15 hours depending on load. The battery cannot be reused after a single use, and is harmful to the environment, so direct sea (marine) application is not possible.
Most of the known seawater- or salt water-activated batteries and electricity generating devices use magnesium as an anode, while the cathode can be made of several different metals (such as stainless steel, copper, titanium), as well as of further compounds such as silver chloride, copper ionide, copper thiocyanate, lead chloride, sodium chloride and copper chloride. These substances are usually either directly or indirectly harmful to sea environments. After use they have to be treated as environmentally hazardous waste.
Further prior art approaches are disclosed in patent documents. In U.S. Pat. No. 2,555,447 an energy cell is disclosed wherein the carbon rod functioning as the positive electrode is surrounded by a volume of matter also containing powder-like carbon, by way of example, graphite. This material assumes a paste- or slurry consistency once it becomes wetted by water or the electrolyte. The material surrounding the positive electrode is encompassed by a layer permeable to the electrolyte. The anode of the energy cell is realized as a container encompassing the electrolyte.
A similar energy cell is described in U.S. Pat. No. 4,020,247, wherein a central collector rod made of carbon is encompassed by a carbon-containing space part that is bounded by a layer that is permeable to the electrolyte. The carbon-containing material is present in the space part surrounding the collector rod in a slurry or powdered form. In this energy cell, also seawater can be applied as an electrolyte.
The common disadvantage of these approaches is that the material present around the collector rod in a powder form becomes slurried. The disadvantages associated with the application of powder-form materials will be described later, in the section related to our experiments.
In US 2015/0037709 A1 an energy cell is disclosed wherein the collector rod is made of graphite and it is surrounded by natural carbon. By way of example, seawater or salt water is applied as an electrolyte in the energy cell. The document does not contain any information as to the shape or geometric dimensions of the natural carbon present in the cell.
In U.S. Pat. Nos. 5,053,375 and 4,885,217 energy cells are disclosed which comprise a single layer of carbon fibres arranged around the cathode. The diameter range of these carbon fibres is between 5 and 15 μm. In the documents reference is made to the application of carbon particles smaller than that.
In U.S. Pat. No. 4,822,698 an energy cell operable by seawater is disclosed that comprises a cathode layer also comprising carbon (powdered carbon). This layer is surrounded by a wall permeable to the seawater electrolyte.
In U.S. Pat. No. 3,849,868 an energy cell is disclosed that comprises a central collector rod, with such a mixture being arranged in the surrounding volume that contains finely divided carbon in addition to the electrolyte. This substance is arranged in a closed container with walls that are not permeable to the electrolyte.
A similar approach is disclosed in U.S. Pat. No. 3,708,344, wherein the material surrounding the collector rod comprises a high percentage of carbon. Such an approach is disclosed in U.S. Pat. No. 2,874,079. A seawater-activated energy cell comprising a carbon cathode is disclosed in JP 60254571 A2.
In U.S. Pat. No. 4,063,006 an energy cell is disclosed which has a cathode comprising carbon and in which seawater is applied as an electrolyte, but the electrolyte does not come into contact with the cathode. Instead, a liquid reagent is brought into contact with the cathode, the reagent being discharged through the same conduit as the seawater functioning as electrolyte. Cathodes comprising activated carbon are mentioned in US 2014/0062382 A1 and WO 89/11165 A1.
In U.S. Pat. No. 3,892,653 a method for generating hydrogen is disclosed wherein magnesium is applied as anode and a carbon rod having similar dimensions as the anode is applied as cathode. The electrolyte is salt water or seawater. The approach according to this document was not developed for generating electricity, but for bringing about a chemical reaction based on the electrode potential difference between the anode and the cathode, using a so-called “short circuit connection”. During the reaction hydrogen is generated on the anode and the cathode in the form of bubbles, the hydrogen being removed from the system by circulating the electrolyte.
Apparatuses similar to the above cited energy cell approaches applying a carbon-comprising material in powdered form around the carbon-based collector rod have been tested in our experiments described below.
During the experiments we have tested the application of a mixture of different carbon types and other additives in the cathode. As in many of the documents describing known approaches, the terms ‘cathode’ and ‘cathode arrangement’ are meant to cover, in addition to the collector rod, also the conductive material arranged around it. In the three experiments described below, the material AZ63 was used as anode (an alloy of magnesium (91% m/m), aluminium (6% m/m), and zinc (3% m/m)).
In a first experiment a mixture of powdered graphite (diameter: d=0.01 mm; typical deviation ±10-15%) and manganese dioxide (MnO2 with IV oxidation capability) in a 70-30 weight % ratio. A cathode housing with a volume of 63 cm3 that will be described in detail below together with sizes of the collector member was applied in the experiment. The following results has been obtained with this material:                The extractable output voltage was 1.125 V without load.        In a seawater environment (applying seawater as electrolyte), under continuous load the output power decreased drastically over a short period of time due to the rapidly emerging polarization. Accordingly, it is not stabile.        During the operation of the energy cell hydrogen generates in the utilized powdered material. Physical/mechanical separation of this gas is not possible, only chemical depolarization.        The cell cannot be regenerated (due to the gas bubbles getting stuck), and the achievable power is low.        
In a second experiment a mixture comprising 70 weight % of powdered graphite (average diameter: d=0.01 mm), 29.25 weight % of manganese dioxide (MnO2(IV)), and 0.75 weight % of nano-carbon (MWCNT—multi-walled carbon nanotubes). Experiments carried out with this material yielded the following results:                The extractable output voltage (1.245V) was slightly higher than in the previous experiment.        In a seawater environment, under continuous load the output power decreases drastically over a short period of time due to the rapidly occurring polarization. Accordingly, it is not stabile.        Physical/mechanical gas separation is not possible, only chemical depolarization.        It cannot be regenerated, and the achievable power is low.        
In a third experiment a mixture comprising 70.175 weight % of powdered graphite (average diameter: d=0.01 mm), 27.569 weight % of manganese dioxide (MnO2(IV)), 1.5 weight % of nano-titanium (TINT: titane nanotubes,) and 0.756 weight % of nano-carbon (MWCNT).                The extractable output voltage increased further to 1.436V.        In a seawater environment, under a continuous load of 50 mA and 100 mA the output power decreases drastically over a short period of time due to the rapidly establishing polarization.        Physical/mechanical gas separation is not possible, only chemical depolarization.        It cannot be regenerated, and the achievable power is low.        The system is capable of stable operation for a relatively short time only with a very low load.        The energy density achieved was: 0.005063 W/cm3        
As it will be shown in FIG. 9 below, with certain geometric configurations the output voltage falls near 0V in a certain period of time; during the present experiments applying nano-carbon a voltage curve similar to that (approximating 0V) was measured. With powdered materials the disadvantageous effect of slurrying is always strongly present.
According to our experiments the application of powder-structure materials in a seawater environment is not expedient because the produced hydrogen gas cannot successfully escape. The application of nano-carbon highly increases the specific surface of the cathode material, but also binds hydrogen molecules to itself, and thereby increases the internal resistance of the cathode. The presence of hydrogen causes polarization, which is one of the main causes of reduced performance.
In accordance with the above, we have found that long-term power generation is not feasible applying powdered materials.
The sustainability of generated current is jeopardized by polarization occurring in the galvanic energy cell. This phenomenon is caused by hydrogen bubbles produced during the chemical processes involved in electricity generation; the positive electrode getting covered by the bubbles, first only partially but later completely, i.e. the presence of hydrogen gas over the cathode surface results in the reduction of the cathode's performance. The equation describing the corresponding reduction process in seawater is:2H2O+2e−→H2+2OH−
The presence of hydrogen first weakens the electric current, and may later completely cut the flow of electrons. This phenomenon can be eliminated by the application of depolarization.
In galvanic energy cells depolarization can be carried out in two ways:                1. Chemically, by applying compounds with high oxygen content, such as manganese dioxide (MnO2 (IV)).        2. Physically, by mechanical gas separation. This latter type of depolarization can be termed ‘forced depolarization’.        
As it is presented below, our tests have confirmed that in a seawater environment chemical depolarization helps only for a very limited period of time. The chemical reaction taking place in seawater-activated energy cells results in the generation of a significant amount of hydrogen gas. Hydrogen can be bound by high oxygen-content compounds as long as there is oxygen surplus in the compounds. When the compounds cannot bind any further hydrogen, the depolarization process stops. In contrast to conventional galvanic cells, a significantly larger amount of hydrogen is generated per unit time from seawater due to its composition, and thereby in this case the applied depolarization compounds become saturated in a much shorter period of time.
Therefore, the efficient depolarization of energy cells applied in seawater environments is only feasible by means of mechanical gas separation.
In a powdered or granulated form (which is an easily crumbling, small particle-size material) the activated carbon applied in the cathode becomes slurried in seawater, which results in that the electrically conductive connections between the particles terminates. In order to prevent this, the activated carbon has to be compacted applying high pressure forces, so that the electrically conductive connections are reinforced by increasing the surface area in contact. However, the size of the passages between the particles of the compacted material thus obtained (which can even be capillary-type, i.e. such passages wherein the flow of the liquid is governed by capillary forces) is reduced by such an extent that these passages are not large enough to allow for the mechanical separation of the generated gases. The experiments have confirmed that hydrogen produced in such energy cells is removed only from the outside layers of the anode, while it gets accumulated in the inside.
In light of the known approaches the need has arisen for a cathode arrangement and an energy cell applying the cathode arrangement that can be operated utilizing seawater or salt water at a stable output voltage for a long time, wherein forced depolarization can be applied with high efficiency.